Method for drying and producing microporous particles and a drying device

ABSTRACT

In a process for drying microporous, fluid-containing particles, the heat required for increasing the temperature is supplied by convection by reducing the interfacial tension of the fluid, preferably to 0 to 1/10, in particular to 0 to 1/20, of the interfacial tension of the fluid at room temperature, by appropriately increasing the temperature at from close to the critical pressure to supercritical pressure of the fluid. Furthermore, microporous, three-dimensionally networked particles are prepared by a process in which the drying process is used. In addition, an apparatus is used for carrying out the drying process, the apparatus comprising a pressure container having an inner container and pressure-withstanding outer container and suitable measuring and control apparatuses and pump apparatuses and heat exchangers, the inner container being provided for holding the particles to be dried and a gap being provided between the inner container and the outer container.

The present invention relates to a process for drying microporous,fluid-containing particles, a process for the preparation ofmicroporous, three-dimensionally networked particles in which thisdrying process is used, and an apparatus for carrying out the dryingprocess.

It is known that hydrogels, e.g. silica hydrogels, which can be preparedby precipitating a gel from waterglass, can be dried under supercriticalconditions to give microporous, three-dimensionally networked silicaparticles. During this supercritical drying, the interfacial tension ofthe fluid contained in the microporous, three-dimensionally networkedparticles is completely or substantially eliminated, with the object ofsubstantially avoiding shrinkage of the microporous, three-dimensionallycrosslinked particles during the drying, since characteristic propertiesof the microporous, three-dimensionally crosslinked particles arecompletely or partly lost in the case of shrinkage. In the case of gels,such a product obtained by supercritical drying is referred to as anaerogel. In contrast to the conventional drying without particularmeasures, during which the gels suffer a large volume contraction andxerogels form, only a small (<15%) volume contraction thus occurs ondrying close to the critical point.

The prior art for the preparation of aerogels by means of supercriticaldrying is described in detail, for example, in Reviews in ChemicalEngineering, Volume 5, No. 1-4, pages 157-198 (1988), in which thepioneering work of Kistler is also described.

In the known processes for the preparation of an aerogel, the requiredheat for circumventing the two-phase region of the fluid contained inthe pores of the particles to be dried is supplied by conduction throughthe container wall (cf. Reviews in Chemical Engineering, Volume 5, No. 1to 4 (1988); Ind. Eng. Chem. Res. 30 (1991), 126-129; and Journal ofMaterials Science 29 (1994), 943-948). It is known that the wall/volumeratio becomes more unfavorable with increasing container volume,correspondingly increasing the batch times on scale-up. Furthermore, thethickness of the pressure-resistant container wall increases with thecontainer diameter. In the case of heat supply externally into athick-walled container under pressure, thermal stresses in the containerwall limit the temperature difference between the inner surface andouter surface of the pressure-resistant container, so that the specificheat supplied by unit volume (watt/m³) into the pressure-resistantcontainer is additionally reduced.

WO-A-95 06 617 relates to hydrophobic silica aerogels, which areobtained by reacting a waterglass solution with an acid at a pH of from7.5 to 11, substantially removing ionic components from the resultinghydrogel by washing with water or dilute aqueous solutions of inorganicbases while keeping the pH of the hydrogel at from 7.5 to 11, displacingthe aqueous phase contained in the hydrogel by an alcohol and thensubjecting the alcogel obtained to supercritical drying.

A process for the preparation of a silica aerogel on the pilot scale isdescribed by White in Industrial and Engineering Chemistry, Volume 31(1939), No. 7, pages 827-831, and in Trans. A. J. Chem. E. (1942),435-447. The process comprises the following steps: preparation andaging of a silica hydrogel, comminution of the hydrogel to givegranules, separation of the salt from the gel formed, replacement of thewater in the gel by alcohol, introduction of the gel, dried so that itno longer drips, into a pressure-resistant container, heating of thepressure-resistant container, reduction of the pressure to atmosphericpressure, evacuation of the pressure-resistant container and subsequentremoval of the aerogel. The disadvantage of this process is that allsteps are carried out batchwise and are thus very time-consuming,labor-intensive and expensive. White does not mention any continuousprocesses for the preparation of granules or for the removal of salt. Inthe water/alcohol exchange, White prefers, for the liquid phase, aprocedure to be described as “covering with alayer/impregnation/drainage”, which procedure constitutes intermittenttreatment of the solids bed with liquid. White believes thatflow-through uniformly as a function of time is less economical.

According to U.S. Pat. No. 3,672,833, the known processes for theremoval of salt from gels and for the replacement of water by othersolvents are extremely tedious and hence expensive processes. Tocircumvent this, this U.S. patent proposes preparing the gel from loweralkyl orthosilicates. However, these require considerable energy intheir preparation.

It is an object of the present invention to provide an improved, moreeconomical process for drying microporous, fluid-containing particles,an apparatus suitable for carrying out this process and an improved,more economical process for preparing microporous, three-dimensionallynetworked particles with the use of the drying process, theabovementioned disadvantages of the prior art being avoided.

We have found, surprisingly, that this object is achieved if the heatrequired for heating to temperatures which are at least close to thecritical temperature of the fluid is supplied by convection. We havefurthermore found that this measure can be carried out particularlyadvantageously in an apparatus in which a pressure container has aninner container and a pressure-withstanding outer container, a gap beingprovided between the inner and the outer container, and the apparatushas suitable measuring and control apparatuses and pumps and heatexchangers. We have furthermore found that microporous,three-dimensionally networked particles can be prepared in aparticularly advantageous manner if, in addition to the use of theabovementioned drying process, any required washing and/or removal ofsalt or a fluid exchange in the pores of the microporous particles andany required removal of sorbed gases or substances are carried out ineach case in a moving bed by the countercurrent method.

The present invention therefore relates to a process for dryingmicroporous, fluid-containing particles by reducing the interfacialtension of the fluid, preferably to 0 to 1/10, in particular to 0 to1/20, of the interfacial tension of the fluid at room temperature, byappropriately increasing the temperature at from close to the criticalpressure to supercritical pressure of the fluid. The novel processcomprises supplying the heat required for the temperature increase byconvection.

The present invention also relates to an apparatus for carrying out thisdrying process, which comprises a pressure container having an innercontainer and a pressure-withstanding outer container and suitablemeasuring and control apparatuses and pump apparatuses and heatexchangers, the inner container being provided for holding the particlesto be dried and a gap or space being provided between the innercontainer and outer container.

The region in which the procedure is preferably carried out according tothe invention can be defined by the fact that the microporous particlesdo not lose their properties during the drying; this means that, forexample, the apparent density of the product does not increasesignificantly, that the thermal conductivity of the product does notincrease significantly, and that preferably no shrinkage above 15%, inparticular above 10%, occurs. This situation can also be described bythe fact that the aerogel may not become a xerogel (gel dried atatmospheric pressure).

The abovementioned interfacial tension is determined as described in TheProperties of Gases and Liquids by Reid, Brausnitz and Sherwood, McGrawHill, 1977, page 601 et seq., the interfacial tension at the temperature(and pressure) to be tested and that at room temperature and atmosphericpressure being measured under otherwise identical conditions andcompared.

In a further embodiment, the invention relates to a process for thepreparation of microporous, three-dimensionally networked particles by

(a) preparing microporous particles containing pore liquid or fluid,

(b) if required, washing and/or removing salt from the particlesobtained in stage (a) and containing pore liquid, by means of a solventand/or water,

(c) if required, partially or completely exchanging the pore liquid orthe solvent or the water in the particles for a fluid to obtainmicroporous, fluid-containing particles,

(d) drying the microporous, fluid-containing particles and

(e) if required, separating off sorbed gases and/or substances from thedried particles from stage (d).

In the novel process, the drying is carried out as described above andstages (b), (c) and (e), if they are carried out, are effected in amoving bed by the countercurrent method by passing the particlesobtained in stage (a) countercurrent to a solvent stream and/or waterstream in stage (b), passing the particles countercurrent to the fluidin stage (c) and passing the dried particles countercurrent to an inertgas stream in stage (e). Preferred embodiments of the invention aredescribed in the following description, the subclaims, the Figure andthe Example.

The only Figure of the attached drawing schematically shows an apparatuswhich is suitable for carrying out the novel drying process.

The microporous, fluid-containing particles which are suitable for thenovel drying are not subject to any particular restrictions per se. Allparticles, solids, structures or granules which are at least partly,preferably wholly, microporous and contain a fluid in the pores aresuitable. Suitable particles are, for example, gels which consist ofinorganic or organic materials or of polymer material, for example ofinorganic oxides or hydroxides, such as boric or silicic acid, oxides orhydroxides of the metals titanium, molybdenum, tungsten, iron or tin oraluminum oxide, or organic gels, such as agar agar, gelatin or albumin.The novel process is particularly suitable for drying silicic acid gels.It is possible to use gels which contain compounds having a criticaltemperature of less than 350° C. or mixtures or conglomerates thereof,preferably water and/or liquid organic compounds, as fluid. Suitablefluids include all compounds which are mentioned below in thedescription of the drying fluids. Particularly suitable fluids arewater, C₁-C₆-alkanols or mixtures thereof, methanol, ethanol, n-propanoland isopropanol being preferred. Isopropanol is most preferred.Depending on the fluid present in the pores, for example, the termshydrogels and alcogels are used. The novel process is most frequentlyused for drying silicic acid gels which contain water, theabovementioned liquid organic compounds or mixtures thereof as fluid.

In a preferred embodiment of the invention, the microporous,fluid-containing particles contain from 50 to 97, in particular from 80to 90, % by weight, based on the total weight of the particles, of fluidunder standard conditions (pressure of 1 bar, temperature of 25° C.).The particle diameters are from 1 to 15 mm, particularly from 2 to 6 mm.Macro-, meso- and/or micropores are present in the particles. Themicroporous particles to be dried may have any shapes, e.g. beads(spheres) or polygonal shapes. The novel drying process is also suitablefor drying microporous, fluid-containing particles or structures whichmay have a certain regular arrangement of the building blocks. Suitableparticles are, for example, also structures crystallized in the presenceof thermally degradable templates, nanostructures whose regulararrangement is self-organized or nanocomposites or their precursors orclathrates. The microporous particles may also be a specifically dopedmicroporous top layer on a nonporous substrate. Catalysts or compoundswhich have acquired chemically reactive centers by impregnation ormodification or are impregnated or modified during drying are alsosuitable. Preferably, aerogels are formed after drying. If the particlesto be dried contain no fluid suitable for the novel drying, said fluidcan be exchanged for a suitable fluid or a more suitable fluid prior todrying. Thus, according to the invention, some microporous particles canbe dried using water as fluid. If, however, it is desired to avoid thehigh critical temperatures and pressures for water as drying fluid,either a water-miscible drying liquid (miscible at least under thedrying conditions), for example an alcohol, may be used or the watercontained in the hydrogel is exchanged wholly or partly for a fluid moresuitable for drying, for example an alcohol. Exchange and drying canalso be carried out simultaneously.

The convective heat supply according to the novel process can beeffected in various ways and is not subject to any particularrestriction. Suitable convection media or streams are all substanceswhich can be brought into the supercritical state without decomposition.Preferably, these are inert to the particles to be dried. In addition,substances may also be added to the convection stream above a certaintemperature, in order chemically to modify, to impregnate or, forexample, to remove traces of water from the structure to be dried.Modification may be desired if, for example, it enables the interfacialtension to be reduced.

Expediently, drying fluids whose critical data is not too high, in orderto avoid more expensive apparatuses, are used as a convection stream ormedium during the drying. Suitable drying fluids are ammonia, sulfurdioxide, nitrogen dioxide and sulfur hexafluoride; alkanes, such aspropane, butane, pentane, hexane and cyclohexane; alkenes, such asC₁-C₇-n-, iso-, neo-, secondary or tertiary alkenes, e.g. ethene orpropene; alkanols, such as methanol, ethanol, n-propanol, isopropanol orbutanols; ethers, such as dimethyl or diethyl ether or tetrahydrofuran;aldehydes, such as formaldehyde or acetaldehyde; ketones, such asacetone; esters, such as the methyl, ethyl, n-propyl or isopropyl estersof formic, acetic or propionic acid; amines, such as mono-, di- andtrimethyl- or -ethyl- or n- or isopropylamine or mixed alkylated aminesthereof; and mixtures of two or more of these fluids. Among said organiccompounds, C₁-C₆-alkanols, -ethers, -ketones, -aldehydes, -alkanes,-alkenes, -esters or -amines are preferred. C₁-C₃-Alkanols are mostpreferred, in particular isopropanol. In principle, halogenatedhydrocarbons are also suitable but are avoided for reasons relating tothe choice of material and environmental requirements. An attempt isalso made to avoid media having high critical temperatures or highpressures, e.g. water. In addition to said drying fluids, supercriticalcarbon dioxide is also suitable for the drying fluid. This isparticularly suitable for thermally sensitive substances, especiallybecause of its advantageous critical temperature of 31° C.

In general, the choice of the drying fluid depends on various points. Ifit is desired to establish near-critical conditions, inter alia thethermal stability of the particles to be dried or of the end productdetermines the choice of the drying fluid and thus also limits thecritical temperature of the drying fluid. In addition, possible fluidrecovery, the toxicological safety, the miscibility with the fluid inthe particles to be dried, the product properties and safety data mayplay a role in the choice of the drying fluid. It is also possible toadd to the drying fluid a component which contains functional groupswhich react or are absorbed or adsorbed onto the surface of theparticles to be dried. Thus, uniform covering, coating or impregnationof the particles to be dried can simultaneously be achieved during thedrying. A modified application of the drying fluid is, for example, theaddition of ammonia to isopropanol as drying fluid, in order, forexample, to be able to dry acidic hydrogels without isopropanoldecomposing. In the case of methanol as drying fluid, the addition ofammonia prevents the undesired formation of a large amount of ether. Forexample, when methanol is used as drying fluid, isopropanol orisobutanol can be added for imparting hydrophobic properties to asilicic acid gel. In general, suitable components can be added before oron reaching the critical temperature of the fluid, for chemical orphysical modification of the particles to be dried.

It is sufficient if the drying fluid is miscible with the fluidcontained in the particles to be dried, at least under the conditionspresent during the drying. Advantageously, however, the drying fluidused is the same as the fluid contained in the microporous particles.Examples of fluids/drying fluids completely miscible under the dryingconditions are mixtures of water with higher alcohols or aromatics.

The convection stream can flow through the bed of the particles to bedried from top to bottom, from bottom to top or outward or in theopposite direction from an axial distributor. Mechanical stability,resilience, particle size distribution and mean diameter of theparticles determine the type of flow through the bed. Any finely dividedmaterial formed can be entrained in the fluid circulation or separatedoff. The bed may be wholly or partly fluidized in the event of flow frombelow. The convection stream can be circulated using a heat-resistantpump or only fresh drying fluid is brought to the required temperaturein a straight pass.

In a preferred embodiment of the invention, the drying is carried out insuch a way that first the convection medium is fed at atmosphericpressure into the drying space and then the particles to be dried, whichare preferably heated, are washed in at atmospheric pressure. Thepressure in the drying space is then brought to the desired value closeto the critical point. Plug flow is then preferably established with theconvection medium. The temperature is then increased until it is closeto the critical point. After near-critical or supercritical conditionsof the fluid have been reached, the pressure is let down, with theresult that the particles are dried. The convection medium can becirculated.

White (Industrial and Engineering Chemistry 31 (1939), No. 7, 827 to831; Trans. A. I. Chem. E. (1942), 435 to 447) proposes, in a batchprocess, discharging the liquid in the void volume before the beginningof the drying step. This proposal can be combined with the novelconvective heat supply.

The interfacial tension of the liquid contained in the pores of theparticles to be dried can also be reduced by adding surface-activesubstances or by prior modification of the microporous, fluid-containingparticles by, for example, silanization, organic esterification oretherification or, in the case of silica gels, by siloxanization ofvicinal silane-mono/di/triols of the inner and outer surface.

In a further embodiment, the present invention relates to a process forthe preparation of microporous, three-dimensionally networked particlesby stages (a) to (e) defined above.

The preparation of microporous particles containing pore liquid can becarried out continuously by processes known to those skilled in the art.

A wash step for the particles obtained in stage (a) can be carried outif undesired components, such as unreacted starting material orimpurities in the starting material, are to be removed. For thispurpose, the particles from stage (a), in the form of a moving bed, arepassed countercurrent to a solvent, preferably a water-miscible one. Asalt removal step (b) for the microporous particles containing poreliquid or solvent can be provided before, after or simultaneously withthe washing or alone (without washing) if the particles containundesired salts. If such a step is used, it is carried out continuouslyby passing the particles obtained in stage (a) or the particles obtainedafter the washing, in the form of a moving bed, countercurrent to astream of water. A suitable ratio or a suitable setting for the materialstreams of particles to be dried and water or solvent for preparing andmaintaining the moving bed can be determined by those skilled in the artby means of customary tests. The setting depends, inter alia, on theheight of the moving bed, on the internal mass transport in theparticles to be dried and on the fluidization point, i.e. on the densityand particle size or particle size distribution of the microporousparticles to be dried. The water stream or solvent stream is preferablyadjusted so that there is no fluidization and hence no undesiredseparation in the moving bed. Back-mixing on the water side or solventside is lowest if a water or solvent flow velocity close to theloosening point of the moving bed is employed. All types of pumps whichare suitable for conveying particulate material are useful as means ofintroducing and discharging the particles to be dried, modified concretepumps having proven particularly useful.

Surprisingly, it has been found that, even in the case of an unstabledensity stratification on the fluid side, the moving bed process can beused without problems for washing and/or removing salts, i.e. it ispossible to use a procedure in which the microporous particles readilymigrate from top to bottom without conveying means. To maintain theunstable density stratification, the density difference is spread over asufficient moving bed length and a minimum relative velocity isestablished. Furthermore, it was surprising here that an acceptablespecific requirement of displacer component is achieved in comparisonwith a batchwise fixed-bed exchange. Furthermore, it was surprisingthat, on salt removal in a moving bed by the countercurrent method, veryadvantageous material requirements (i.e. required fresh water volume forobtaining a specific volume of hydrogel from which salts had beenremoved) could be achieved. It is all the more surprising since theliterature, as stated above, described the salt removal step as verycomplicated and tedious, for which reason U.S. Pat. No. 3,672,833proposed the hydrolysis of lower alkyl orthosilicates for preparingsilica aerogels.

All desired degrees of wash-out and degrees of salt removal can beestablished. The washing step and/or salt removal step are acceleratedby increasing the temperature, i.e. the higher the temperature thefaster they take place. Preferably, they are therefore carried out atelevated temperatures, the upper limit for the temperature beingpredetermined, inter alia, by the decomposition of the particles to bewashed or from which salts are to be removed, theiragglomeration/tendency to stick together, dissolution in the fluid, etc.For example, salts can be removed from some silica gels at about 80° C.To improve the cross-mixing, pulsation of the solvent or water streamcan also be provided. Furthermore, the moving bed can be loosened bybubbling in gas, e.g. air. Preferably, salts are removed from silica gelin stage (b) after aging.

In stage (c), some or all of the pore liquid contained in the particles,in particular from 97 to 99%, is exchanged for a fluid. Suitable fluidsare the fluids described above in the description of the microporous,fluid-containing particles. Analogous to salt removal, elevatedtemperatures favor the exchange. Regarding the suitable temperature,statements made above under stage (b) are therefore applicable.Furthermore, the statements made above under stage (b) are applicable tothe establishment of the moving bed. In the exchange step, too, alldesired degrees of exchange can be established. Such an exchange of thepore liquid can of course be dispensed with if the particles obtained instage (a) or (b) already contain a suitable fluid. It is also possible,in stage (c), for the pore liquid in the particles first to be exchangedfor a liquid miscible with the pore liquid but not fluid suitable fordrying. In this case, the liquid miscible with the pore liquid is thenexchanged for a fluid suitable for drying. In stage (c), it is alsopossible to feed in material streams of different purities at differentheights. Furthermore, a combination of the exchange step with aseparation of a fine fraction or, for example, adhering oil from thegelling, is possible and may save a separate classification step.Moreover, the combination of salt removal in stage (b) and exchange instage (c) in one apparatus may be advantageous under appropriate kineticconditions. If traces of the original pore liquid present problems inthe exchanged particles, they can be removed in a separate moving bedunder special conditions, for example by a reaction. This is alsopossible by adding suitable components at the bottom of the exchangemoving bed, and a combination with impregnation of the microporousparticles is also possible.

In stage (d), the microporous, fluid-containing particles are dried. Thedrying is carried out by means of convective heat supply, as describedabove in the case of the novel drying process.

In any stage (e) carried out, the dried particles are separated or freedfrom absorptively and/or adsorptively bound gases and/or substances.This step is carried out continuously in the moving bed by thecountercurrent method, the dried particles being fed countercurrently toan inert gas stream, preferably under reduced pressure. Suitable inertgases are nitrogen, carbon dioxide or noble gases. Under certaincircumstances, air or stack gas may also be used. Regarding theestablishment of the moving bed, statements made above under stage (b)apply analogously. It is also possible to add to the inert gas phase acomponent which reacts with the dried particles or is absorbed oradsorbed. The separation step can, if required, be improved byadsorption by displacement with a more strongly adsorbing substance. Insome cases, the removal of the absorptively and/or adsorptively boundsubstances and/or gases can be effected simply by applying reducedpressure.

Stage (e) can be followed by a continuous final treatment step in whichthe microporous, three-dimensionally networked particles are broughtinto the desired form, for example by milling, sieving or mixing withadditives suitable for use. It is also possible to provide the particlesobtained with a hard coat, for example by means of sintering, in orderto increase their mechanical strength.

The microporous, three-dimensionally networked particles obtained arethe same particles as have been described above in the case of the noveldrying process, these particles additionally having been freed fromundesirable secondary substances compared with the abovementionedparticles.

The microporous particles obtainable by means of the novel process canbe used in many industrial areas. They are suitable, inter alia, for thepreparation of transparent or opaque thermal insulation materials (undercertain circumstances as a substitute for chlorofluorocarbon-containingmaterials). They are also used as catalysts and catalyst supports,adsorbents, electrodes (carbon aerogels obtained by coking ofmicroporous polymers, for example in capacitive energy stores whenimpregnated with electrolyte), membranes, Cerenkov detectors, superlightsponges for inclusion/storage or gelling agents/thickeners/thixotropicagents for liquid fuels for space flight, insecticides, sinterableintermediates for ceramics or high-purity optical fibers, piezoceramicoscillators in ultrasonic transmitters, in acoustic antireflectivelayers, as dielectrics, as carriers for fluorescent dyes, as dullingagents, as additives in lubricants, rubber and sealants, in compositematerials and in coatings and lakes.

The novel apparatus for drying microporous, fluid-containing particlescomprises at least one two-shell container comprising inner containerand pressure-withstanding outer container and suitable measuring andcontrol apparatuses and pump apparatuses and heat exchangers. Accordingto the invention, the inner container is provided or intended forholding the particles to be dried, and a gap or space is providedbetween the inner container and outer container. The inner container mayhave any desired shape but is preferably rotationally symmetrical, forexample a cylinder having a conical outflow or a sphere, so that, in apreferred embodiment, the gap is formed rotationally symmetrically. Theinner container may be formed to be conical at the top and/or thebottom. It may be produced from any material which still has therequired strength at the drying temperature to be established. Stainlesssteel, boiler plate or glass fiber-reinforced plastics are preferred.Stainless steel is most preferred. The inner container is preferablythin-walled and is preferably designed for pressures of less than 6 bar.The outer container consists of materials which have the pressureresistance required for drying. Grain-refined structural steel orcreep-resistant steel is preferred. The gap or space between the innercontainer and outer container ensures thermal insulation. It isexpediently filled with an inert gas, preferably a poorlyheat-conducting gas, such as nitrogen or krypton. To improve theinsulation, it can also be filled with insulation material (e.g.rockwool or glass wool).

The Figure describes an apparatus comprising inner container and outercontainer and suitable measuring and control apparatuses and pumps andheat exchangers, which apparatus is particularly suitable for carryingout the novel drying process. The actual dryer consists of thethin-walled inner container 1 and the pressure-withstanding outercontainer 2. The novel process is carried out as follows. First, theinner container 1 is filled with drying fluid via line 3. The particlesto be dried are then washed in from the storage container 4 via line 5at the top of the dryer by means of drying fluid. The dryer is closedand the pressure therein is increased to near-critical to supercriticalconditions. The pump 6 then forces the drying fluid heated in heatexchanger 7 into the particle bed from below. The drying fluid is fedfrom the top of the dryer back to the pump 6 or partly released viavalve 8 to keep the pressure in the dryer constant, until near-criticalto supercritical temperatures have been established throughout thedryer. Valve 8 is then opened. The dried particles are removed via line9. Differential pressure control is preferably used between innercontainer 1 and outer container 2, since the inner container 1 should asfar as possible be constructed with thin walls. This differentialpressure control operates as follows: if the level in storage vessel 10rises because excess pressure is present in the drying fluid circulationand drying fluid flows via the cooler 11 to the storage vessel 10, thepressure of the N₂ cushion in the gap formed between the inner and outercontainer is increased via a level sensor 12 with the aid of an N₂ splitrange control 13. If the level in the storage container 10 drops, the N₂split range control 13 correspondingly reduces the pressure of the N₂cushion in the gap. To avoid introduction of a fine fraction into thestorage container 10, a small cleaning fluid stream 14 is passed to thestorage container 10 via a flow control. This material stream may alsoperform, inter alia, the task of reducing the build-up of troublesomecomponents which form, by feeding fresh fluid. If the control of thedifferential pressure between inner container 1 and outer container 2fails, an overflow valve between inner container and outer container(not shown) preferably protects the inner container 1. To protect theinner container 1, the pressure drop between the bottom and top of theparticle bed should be limited. If corresponding controls fail, theinner container 1 is protected from destruction by a further overflowvalve in a dryer bypass (not shown).

The invention has the advantages that considerable quantities of energyare saved since the outer container is subject to only a smalltemperature change in the course of drying. In addition, the thermalcycling of the flanges and other parts of the apparatus is substantiallyreduced in comparison with known processes from the prior art. In orderto load the dryer, only the inner container has to be cooled, forexample by evaporative cooling. By dispensing with heat-up and coolingprocesses of the outer container, the batch time is considerablyreduced.

Surprisingly, it has been found that, with flow through a bed frombottom to top, as shown in the Figure, a major part of the initiallytaken liquid can be displaced from the container at room temperature. Incomparison with a continuous powder process in which large amounts ofsolvent have to be heated cocurrently with the solid to be dried,further energy is thus saved. Surprisingly, it has furthermore beenfound that conventional silicic acid alcogel granules obtained fromhydrogel are not destroyed or damaged by the convective heat supply,either mechanically or as a result of thermal stresses. This alsoapplies to gel beads in the lowermost layer of the bed, which are stillat ambient temperature and into which fluid at 300° C. flows.

The invention is additionally illustrated in more detail by thefollowing Example, which is a preferred embodiment of the invention.

EXAMPLE

Stage (a): Hydrogel Preparation

Silicic acid hydrogels were prepared according to DE-A-21 03 243,DE-A-44 05 202 and DE-A-16 67 568. At least 95% by volume thereof had abead diameter of from 2 to 12 mm. Coarse materials were separated off bymeans of a wire rod sieve immersed in water. Next, the silicic acidhydrogels were subjected to continuous hydraulic classification prior tosalt removal.

Stage (b): Salt Removal

Apparatus

Two desalination moving beds, each 11 m high and 800 mm wide, wereequipped with sampling points at various heights. Fresh water was fed inat the bottom by distributors and salt water was removed at the top viaslit sieve cartridges. The cellular wheel sluice at the bottom adjustedthe solids streams. At low flow velocities and in the case of gelstending to stick, the cross-mixing in the bed was improved by means ofstatic mixers.

Procedure

In each desalination moving bed, a stream of about 2450 l/h of water wasfed from below countercurrent to a downward-moving stream of about 510l/h of classified hydrogel from the preceding stage (about 150 of the510 thereof are accounted for by the gap volume). After about 30 hoursat the latest, a steady state had been established in the moving bed.The conductivity of the samples which were removed at the various pointsalong the bed no longer showed any changes. A conductivity of more than1 milli-Siemens/cm was measured in the overflow. The water in the gapvolume of the hydrogel from which salt had been removed had aconductivity of 40 micro-Siemens/cm, which corresponds to a sodiumcontent of about 1% by weight in the gel.

Stage (c): Water/Alcohol Exchange

Apparatus

The liquid exchange step was carried out in a moving bed which was 11 mhigh and 500 mm wide and which was designed similarly to that used forthe salt removal. The alcohol was fed in above the cellular wheelsluice, by means of a distributor. The water/alcohol mixture was able toflow away via slit sieves. At low flow velocities and in the case ofgels which tended to stick, the cross-mixing in the bed was improved bymeans of static mixers.

Procedure

About 1400 /h of isopropanol were fed countercurrent to about 1000 /h ofthe hydrogel from stage (b), from which salt had been removed. After 10hours at the latest, a steady state had been established in the movingbed. The densities of the samples from the various sampling points alongthe bed no longer showed any change. The residual water content of thegel which was discharged at the bottom of the moving bed was less than1% by weight. The specific isopropanol demand volume ratio was thus1.4:1.

Stage (d): Drying

Apparatus

The apparatus used corresponded schematically to the apparatus shown inthe Figure. Thus, the apparatus used consisted of a 100 barpressure-resistant outer container of creep-resistant steel, plated withstainless steel on the inside, and a 400 mm wide inner container ofstainless steel. The outer container was 8 m high and cylindrical andhad an external diameter of 600 mm and a wall thickness of 50 mm. Theinner container had a wall thickness of 4 mm and tapered conically atthe top and bottom. The effective volume was 1 m³. The nitrogen-filledannular gap between the inner container and outer container was 50 mmwide in the cylindrical region. The inner container communicated withthe drying fluid circulation, in which pressure control means,circulation pump and heat exchanger were housed. A nozzle, which had thealcogel feed line centrally and the sieve surface for fluid/solidseparation on its outer cylindrical side, projected into the top of theinner container.

Procedure

The pressure-withstanding part of the dryer was heated to 300° C. with100 bar steam. The inner container was evaporatively cooled byisopropanol addition. Alcogel was washed in with isopropanol, which wascirculated. During this loading process, the temperature of the alcogelscarcely increased. After the dryer had been closed, the annular gap andinner container were brought to 60 bar. Regarding details of thepressure control, reference may be made to the Figure. The pump wasswitched on and drying fluid was fed in, initially at low speed, e.g. 1m³ per hour, at a density above 0.7 kg/l. Flow through the alcogel bedwas from below. The heat exchanger was then heated. The pump speed couldbe increased with decreasing density of the drying fluid. Instead of thedensity, the temperature at the top of the dryer could also be used as areference variable. 70% of the isopropanol were displaced from thecirculation at room temperature.

After 50 minutes, the supercritical temperature was reached at the topof the bed. The pressure was let down without affecting the two-phaseregion.

Stage (e): Removal of Sorbed Gases/Substances

Apparatus

A 3 m³ silo was used for removal/separation of the sorbedgases/substances.

Procedure

After the pressure had been let down, the aerogel was transferredpneumatically into the silo. The silo was then evacuated and a gentlestream of nitrogen was allowed to flow through the bed at about 30 mbar.This nitrogen stream exchanged the gas atmosphere in the silo ten timesper hour. Consequently, the partial pressure of desorbed alcohol waskept low and the desorption was accelerated and completed. The residencetime was more than 30 minutes, in order also to remove sorbedgases/substances from the Knudsen pores of the aerogel. If it wasdesired or necessary to cool, the silo was operated at atmosphericpressure and N₂ was circulated via a scrubber.

Treatment

The continuous treatment step was carried out by milling and mixing indopants (blowing in) in a pinned disk mill.

The aerogel granules obtained had a particle size of up to 12 mm, only2% by volume of the granules having a particle size of less than 2 mm.The mean thermal conductivity e₁₀ of the 2-3 mm fraction of the granuleswas better than 18 mW/(m·K) according to DIN 52616, and was 16 mW/(m·K)for the powder. The transparency of the 2-3 mm fraction was 60% for alayer thickness of 1 cm. The bed density according to ISO 3944 was from70 to 130 g/l. The aerogel was water-repellent and floated on water. Theheadspace (the gas phase above the bed) of the aerogel was nonexplosiveat 100° C. and explosive at 160° C. only after one hour.

Surprisingly, it was found that the gel was not damaged in spite of therapid heating up, that the abrasion resistance of the gel was sufficientand that scarcely any water accumulation in the fluid occurred. In somecases, even a reduction in the water content was observed, whichpermitted reuse of the solvent without thermal working-up, withoutaccumulation of water in the drying fluid occurring.

We claim:
 1. A process for drying microporous, fluid-containingparticles by reducing the interfacial tension of the fluid at roomtemperature, by increasing the temperature that exist from close to thecritical pressure to supercritical pressure of the fluid, whichcomprises supplying the heat required for the temperature increase byconvection.
 2. A process as claimed in claim 1, wherein thefluid-containing particles dried are gels which contain water,C₁-C₆-alkanols or mixtures thereof as fluid.
 3. A process as claimed inclaim 1, wherein gels which contain isopropanol as fluid are dried.
 4. Aprocess as claimed in claim 1, wherein silicic acid gels are dried.
 5. Aprocess as claimed in claim 1, wherein a drying fluid is used for theconvective heat supply.
 6. A process as claimed in claim 5, wherein thedrying fluids used are C₁-C₆-alkanols, C₁-C₆-ethers, C₁-C₆-ketones,C₁-C₆-aldehydes, C₁-C₆-alkanes, C₁-C₆alkenes, C₁-C₆-esters orC₁-C₆-amines or carbon dioxide.
 7. A process as claimed in claim 5,wherein the drying fluid used is the same fluid as that contained in themicroporous particles.
 8. A process for the preparation of microporous,three-dimensionally networked particles by (a) preparing microporousparticles containing pore liquid or fluid, (b) optionally washing and/orremoving salt from the particles obtained in stage (a) and containingpore liquid, by means of a solvent and/or water, (c) optionallypartially or completely exchanging the pore liquid or the solvent or thewater in the particles for a fluid to obtain microporous,fluid-containing particles, (d) drying the microporous, fluid-containingparticles and (e) optionally separating off sorbed gases and/orsubstances from the dried particles from stage (d), wherein the dryingin stage (d) is carried out as defined in claim 1 and stages (b), (c)and (e) are carried out in a moving bed by the countercurrent method, bypassing, in stage (b), the particles obtained in stage (a)countercurrent to a solvent stream and/or water stream, passing theparticles countercurrent to the fluid in stage (c) and passing the driedparticles countercurrent to an inert gas stream in stage (e).
 9. Anapparatus for carrying out the drying process as claimed in claim 1which comprises a pressure container having an inner container and apressure-withstanding outer container and measuring and controlapparatuses and pump apparatuses and heat exchangers, the innercontainer being provided for holding the particles to be dried and a gapbeing provided between the inner container and the outer container. 10.An apparatus as claimed in claim 9, wherein the inner containercomprises stainless steel and the pressure-withstanding outer containercomprises creep-resistant steel.
 11. The process as claimed in claim 1,wherein the interfacial tension of the fluid is reduced to 0 to 1/10 ofthe interfacial tension of the fluid at room temperature.
 12. Theprocess as claimed in claim 1, wherein the interfacial tension of thefluid is reduced to 0 to 1/20 of the interfacial tension of the fluid atroom temperature.